Antenna array switchable to provide spatial shift without change of radiation pattern

ABSTRACT

A receiver comprising an antenna array having a number of antenna elements. The antenna array is fed by a feeder. Switching apparatus are arranged for switching at least a first antenna element to the feeder or at least a second antenna element to the feeder. This provides a spatial shift of the antenna array without changing its radiation pattern. This improves the signal level when poor signal reception occurs due to multipath fading.

BACKGROUND OF THE INVENTION

This invention relates to a receiver for receiving radio signals, havingan antenna array comprising a plurality of antenna elements and a feedfor feeding the antenna elements.

The invention also relates to an antenna array for such a receiver.

Receivers comprising antenna arrays are widely known. They can be usedin situations in which a large gain factor in a specific direction iswanted. One of these situations is in a system for cellular TV. In sucha system a transmitter or a small number of transmitters per celltransmit signals. The receivers are directed to one of the transmittersin order to receive the signal well.

A problem which exists especially in urban environments is the problemof multipath fading. This means that signals due to reflections reachthe antenna via a plurality of different paths. Due to transmission timedifferences between the signals, there will be phase differences betweenthem. Because of this, there will be some points in which the signalsare added and some points in which the signals are cancelled. Thereforean interference pattern of the received signal power level with maximaand minima (nulls) will occur. When the antenna array is in or near aminimum this will result in a bad signal reception.

The problem of multipath fading is also known from other fields, e.g.cordless telephony. A solution known from that field is to have twoantennas separated by a certain distance. In the situation where thefirst antenna is switched on and the received signal is bad, the antennaprobably is near a minimum. By switching it off and switching the secondantenna on, almost always a better reception can be obtained.

However, this is not an attractive solution for a receiver with anantenna array since the duplication of a whole array is awkward andexpensive. Furthermore, this solution takes a relatively large amount ofspace.

SUMMARY OF THE INVENTION

The present invention has as an object to provide a receiver having anantenna array, which also when multipath fading occurs, has a goodreception and which is simple and cheap.

Thereto a receiver according to the invention is characterized in thatthe antenna array comprises switching means for switching at least afirst antenna element to the feed or at least a second antenna elementto the feed for obtaining spatial shifting of the antenna array withoutchanging its radiation pattern. So, a spatial shifting is obtainedwithout having to provide a whole second antenna array. The radiationpattern should not be changed in order to keep the antenna well directedto the signal source.

An embodiment of a receiver according to the present invention ischaracterized in that the array comprises a number of columns ofantenna-elements, and in that the switching means are arranged forswitching a first edge column to the feed or switching a second edgecolumn to the feed, the second edge column being opposite to the first.By switching between two opposite edge columns a large spatial shiftingis obtained very easily.

A further embodiment of a receiver according to the present invention ischaracterized in that the receiver comprises detection means fordetecting a decrease of a received signal strength below a thresholdvalue and in that the switching means are arranged for automaticallyswitching when the detection means detect such a decrease. When thesignal strength falls below a threshold this means that the antennaarray is positioned at a point where the signal strength is small. Byswitching the switching means in their other state, very probably, thereceived signal strength will increase.

A further embodiment of a receiver according to the present invention ischaracterized in that the antenna array is a planar array and in thatthe antenna elements are patch antennas. In this way a very manageablestructure of the antenna array is obtained.

BRIEF DESCRIPTION OF THE DRAWING

The invention now will be described with reference to the accompanyingdrawing in which:

FIG. 1 shows a simplified block diagram of a cellular television system,

FIG. 2 shows the occurrence of multipath fading,

FIG. 3 shows a first preferred embodiment of a receiver according to theinvention, and

FIG. 4 shows a second embodiment of a receiver according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a simplified block diagram of a cellular television system.Such a system is known from the European Patent Application No.0,282,347. Herein a cellular television system is described. The systemcomprises an array of similar cells. Only one cell CL is indicated inthe figure. In order to cover the complete cell CL, a transmitter 1,located in the centre of the cell CL, is provided with anomnidirectional antenna 15, e.g. implemented by a small number ofquadrant directional radiators. A multiplexed signal is modulated onto amicrowave carrier so as to allow a wide variety of signal formats to becombined efficiently. Such signals can be FM (Frequency Modulation)video signals with audio sub carriers, digital signals or the like. Theband in which the signals are transmitted goes from 27.5 GHz to 29.5GHz. The system utilizes relatively low power transmitters so as toallow for frequency reuse in a given geographical area.

The receivers 3 in the system are to be supplied with antennas 4 whichhave a large gain factor in the direction of the transmitter. Thisallows the transmitted signal power to be as low as possible. An antennawith a large gain factor can, for example, be a microstrip antennaarray.

FIG. 2 shows in a simplified way the occurrence of multipath fading. Thereceiver 3 finds itself near a reflective body 2, e.g. a wall. There isone signal path SP1, directly from the transmitter 1. Another signalpath SP2 goes via the wall to the receiving antenna 4. Due to this therewill be a phase difference between the signals coming in via therespective signal paths SP1 and SP2. Depending on the size of this phasedifference this can lead thereto, that the respective signals canceleach other or add to each other. In case of multipath fading aninterference pattern will be present, consisting of points in which thesignals add to each other (maxima) and points in which the signalscancel each other (minima/nulls). The distance between those points isstrongly dependent on the specific situation. Of course FIG. 2 shows theoccurrence of multipath fading in a simplified way. Additionalreflective bodies can be present, more signal paths can play a role,etc. Furthermore the interference pattern can change rapidly: Passingcars in the street, birds, even people moving within the house, allthese factors have their influence.

When the antenna finds itself in a point of the interference patternwith a minimal strength (minimum) the signal reception will be bad.Moving the antenna about a wavelength, normally will give a much betterreception.

FIG. 3 shows a first preferred embodiment of a receiver according to theinvention. The receiver comprises a microstrip antenna array 4. Theantenna array comprises antenna elements 5-1 . . . 5-20. These elementsare patch antennas. As is widely known, they work in the best way whenthey have a length which is a bit smaller than half a wavelength in freespace. When a signal frequency of 29 GHz is used, the length of theantenna elements must be about 6 or 7 mm. The exact length required forthe patch antennas depends on the properties of the dielectric materialwhich is used. The antenna elements are arranged in five columns 6-1 . .. 6-5 of each having four antennas. The distance between the antennaswithin the column is approximately half a wavelength. The distancebetween the columns is also about half a wavelength. The antenna is fedby a feeder 7-1. All the columns are fed with the same phase. All theantennas within the columns are also fed with the same phase. Theantenna array comprises switching means 8-1, 8-2. The switching meansare arranged for switching either column 6-1 or column 6-5 to the feeder7-1. When column 6-1 is switched to the feeder a normal 4×4 microstripantenna array is obtained, consisting of the elements 5-1 to 5-16. Thisantenna array has a centre point 11 (indicated by a dot). When column6-5 is switched to the feeder another normal 4×4 microstrip antennaarray is obtained, consisting of the elements 5-5 to 5-20. This antennaarray has a centre point 12 (indicated by a cross). So, only a spatialshift of the antenna array is obtained. Its radiation pattern is notchanged by the switching action.

The receiver comprises furthermore down converter means 10 and detectionmeans 9. The detection means are arranged for detecting the occurrenceof bad signal reception due to multipath fading in a well known way.They calculate the ratio between the signal and the out of band noise.When this ratio falls below a certain threshold value, the presence ofmultipath fading is assumed. The detection means then switch theswitching means to their other state. Normally this will result in amuch better reception due to the spatial shift of about a wavelengthbetween the centre points 11 and 12.

The down converter means convert the 29 GHz received signal into asignal with a much lower frequency which is transferred to a TV.

The switching means can be arranged in any suitable way. For example,PIN-diodes can be used.

FIG. 4 shows a second embodiment of a receiver according to theinvention. The receiver 3 comprises an antenna array 4 consisting offive antenna elements 5-21 . . . 5-25 fed by a feeder 7-2. The switchingmeans 8-1,8-2 are arranged to switch either the first antenna element5-21 or the fifth antenna element 5-25 to the feeder 7. When antennaelement 5-21 is switched to the feeder an antenna array is obtainedconsisting of the elements 5-21 to 5-24. This antenna array has a centrepoint 31 (indicated by a dot). When antenna element 5-25 is switched tothe feeder a microstrip antenna array is obtained consisting of theelements 5-22 to 5-25. This antenna array has a centre point 32(indicated by a cross).

It will be understood that the embodiments shown are only meant asexamples. The invention is not limited to receivers in a cellulartelevision system, but it can be applied to all kind of receivers ofradio signals. Also the implementation of the antenna array is notessential: e.g. the antenna elements can be dipoles instead of patchantennas, arrays with different numbers of columns can be used, threedimensional arrays can be used etc. Those modifications will be apparentto persons skilled in the art. The distinguishing feature of theinvention is: spatial shifting within an antenna array by switchingbetween antenna elements, without changing the radiation pattern of thearray.

I claim:
 1. Apparatus for receiving radio signals comprising:a. anantenna array including a group of antenna elements, at least one firstantenna element and at least one second antenna element; b. a feederapparatus for coupling selected ones of the antenna elements to autilization device; and c. a switch apparatus for selectivelyconnecting:(1) the at least one first antenna element and said group ofantenna elements, as a first subarray, to the feeder apparatus; or (2)the at least one second antenna element and said group of antennaelements, as a second subarray, to the feeder apparatus; said first andsecond subarrays being spatially separated centers, but havingsubstantially-identical radiation patterns.
 2. Apparatus as in claim 1where the array comprises a number of columns of antenna elements, saidat least one first antenna element comprising a first column of saidelements at a first end of the array and said at least one secondantenna element comprising a second column of said elements at a secondend of the array.
 3. Apparatus as in claim 2 where the first and secondcolumns of antenna elements are at opposite ends of the array. 4.Apparatus as in claim 1, wherein the apparatus comprises detection meansfor detecting a decrease of a received signal strength below a thresholdvalue, and the switching means are arranged for automatically switchingwhen the detection means detect such a decrease.
 5. Apparatus as inclaim 1 wherein the antenna array comprises a planar array and theantenna elements comprise patch antennas.
 6. Apparatus as in claim 1,wherein the apparatus comprises detection means for detecting a decreaseof a received signal strength below a threshold value, and the switchingmeans. are arramged for automatically switching when the detection meansdetect such a decrease.
 7. Apparatus for receiving radio signals,comprising:an antenna array comprising a plurality of antenna elementsarranged to provide a radiation pattern, a feeder apparatus for couplingthe antenna elements to a utilization device for received radio signals,and switching means for selectively switching at least first and secondantenna elements of the antenna array to the feeder apparatus in amanner so as to provide a spatial shift of a centerline of the radiationpattern of the antenna array and without change of the radiation patternof said antenna array, said apparatus comprising a single antenna arrayin which the switching means in a first position connect and disconnectthe first and second antenna elements to the feeder apparatus at thesame time so as to align the radiation pattern with a first centerlinethereof, and in a second position thereof disconnect and connect thefirst and second antenna elements to the feeder apparatus at the sametime so as to align the radiation pattern with a second spaced apartcenterline of the radiation pattern.
 8. Apparatus for receiving radiosignals, comprising:an antenna array comprising a plurality of antennaelements arranged to provide a radiation pattern, a feeder apparatus forcoupling the antenna elements to a utilization device for received radiosignals, and switching means for selectively switching at least firstand second antenna elements of the antenna array to the feeder apparatusin a manner so as to provide a spatial shift of a centerline of theradiation pattern of the antenna array and without change of theradiation pattern of said antenna array, said antenna elements beingarranged in a plurality of columns with at least one column of antennaelements directly connected to the feeder apparatus, at least first andsecond other columns of antenna elements connected to the feederapparatus via the switching means, said switching means connecting thefirst and second other columns of antenna elements to the feederapparatus at mutually exclusive times.
 9. The receiving apparatus asclaimed in claim 8 wherein said first and second other columns ofantenna elements comprise the respective outer edge columns of theplurality of columns of antenna elements forming the antenna array. 10.The receiving apparatus as claimed in claim 8 comprising a signalantenna array wherein each column of antenna elements has the samenumber of antenna elements which are spaced apart by half a wavelengthand the columns of antenna elements are spaced apart by a halfwavelength.